The present invention relates to a method and an apparatus for forming an inflation film and, more particularly, to a method and an apparatus for forming an inflation film by air-cooling a thermoplastic resin having a melt tension of not more than 4 g.
Low or medium pressure process thermoplastic resins such as homopolymers of ethylene or copolymers thereof with .alpha.-olefins generally have a smaller melt tension than, for example, high pressure process low-density polyethylenes. The thermoplastic resins which have a small melt tension have a narrow molecular-weight distribution and a smaller number of long-chain branches. For these reasons, when a bubble (tubular film) is formed from a melted thermoplastic resin having such a small melt tension in the manufacturing step of an inflation film, it has poor resistance to air pressure, that is, poor shape retaining force. More specifically, when air is blown toward a bubble at a relatively high velocity, the bubble may flap or sway, or necking may occur, which results in producing a defective product or preventing further forming.
In a conventional method for molding an inflation film by forming a thermoplastic resin having a small melt tension, the velocity of a gas flow for cooling cannot be increased above a predetermined value, unlike in the case of a high pressure process low-density polyethylene. Therefore, the forming rate is extremely low, that is, the productivity of the inflation film is low.
When a film of 30 to 50 .mu. in thickness is formed from a high pressure process low-density polyethylene, the forming speed is generally about 50 to 60 m/min. However, when a film of the same thickness is to be produced from a thermoplastic resin having a melt tension of not more than 4 g, only a maximum molding speed of about 20 m/min may be obtained. If the thermoplastic resin having a small melt tension is formed at a speed exceeding about 20 m/min, the air velocity for cooling the bubble must also be increased. However, if the air velocity for cooling is increased during forming of the thermoplastic resin having a small melt tension, the bubble may flap or a neck may be formed in the bubble, which results in defective products or prevention of further forming.
FIG. 1 shows one conventional method for forming an inflation film. According to this method, in order to cool the bubble, air is blown from an external air-cooling ring 14 along the outer circumferential surface of a bubble 12, which is taken off from a die 10. When the air velocity toward the bubble 12 is increased in this method, a neck 16 is formed in the bubble to decrease the heat-exchange area (area for exchanging heat). In some cases, on increase in the air velocity toward the bubble 12 results in prevention of further forming. Thus, the air velocity cannot be increased above a predetermined value. Since it takes a considerable amount of time to cool the bubble, the forming rate of a thermoplastic resin having a small melt tension cannot be increased. In order to form a film of 50 .mu. thickness from an ethylene-.alpha.-olefin copolymer, a maximum air velocity is 15 m/sec and a maximum molding speed is 20 m/min.
FIG. 2 shows another conventional method wherein an air-cooling ring 18 is arranged inwardly of a bubble 12. Air from the air-cooling ring 18 is blown along the inner circumferential surface of the bubble 12 so as to cool the bubble 12 which is being taken off from a die 10. In this conventional method, if the air velocity is increased to exceed a predetermined value, the interior of the bubble is kept at a reduced pressure to form a neck 16 again. Thus, the air velocity for cooling the bubble cannot be increased above a predetermined value in this method, either. In other words, the forming rate of the thermoplastic resin having a small melt tension cannot be increased. In order to form a film of 50 .mu. thickness from an ethylene-.alpha.-olefin copolymer, a maximum air velocity of 14 m/sec and a maximum forming rate is 15 m/min.
FIG. 3 shows yet another conventional method wherein a combination of the external cooling method shown in FIG. 1 and the internal cooling method shown in FIG. 2. Even with this conventional method shown in FIG. 3, a maximum forming rate of only 35 m/min may be obtained, which is the sum of the maximum forming rate of the external cooling method which is 20 m/min and the maximum forming rate of the internal cooling method which is 15 m/min. If the forming rate is increased above 35 m/min in this method by increasing the air velocity, a sharp neck is formed to make any further resin forming impossible.
According to yet another conventional method shown in FIG. 4, a cylindrical guide plate 20 is arranged outside a bubble 12. According to this method, the inside of the guide plate 20 is subject to a reduced pressure as the velocity of air for external cooling is increased. This reduced pressure causes flapping of the bubble 12 to cause the bubble 12 to contact with the guide plate 20. For this reason, the air velocity cannot be increased; thus the forming rate cannot be increased. In order to form a film of 50 .mu. thickness from an ethylene-.alpha.-olefin copolymer, a maximum forming rate is 30 m/min.
The air velocity used herein is a value measured with a hot-wire anemometer at the blowing port of the air-cooling ring.
The melt tension used herein is a physical value which is measured with a "melt tension tester" type II (product manufactured by Toyo Seiki Seisakusho K.K.) under the following conditions:
Nozzle shape: 2.09 mm diameter; 8.0 mm length PA1 Piston transfer speed: 15 mm/min PA1 Extrusion temperature: 190.degree. C. for ethylene resin; PA1 Take-off speed: 45 m/min PA1 Tension measurement point: 500 mm below the nozzle PA1 (a) An increase in the heat-exchange area. PA1 (b) An increase in the velocity of the cooling air to be blown against the bubble. PA1 (c) Stabilization of the bubble; prevention of formation of a neck or flapping.
230.degree. C. for propylene resin
According to this measurement method, the tension of a thread-like resin body which is taken off from a nozzle is measured with a tension tester at a point 500 mm below the nozzle.